Single Photon Emission Computed Tomography (SPECT) is a nuclear-medicine imaging modality capable of providing 3D maps of in vivo radiopharmaceutical distributions. It is very similar to conventional nuclear medicine planar imaging, but in SPECT a gamma ray camera is used. In the same way that a plain X-ray is a 2-dimensional (2D) view of a 3-dimensional (3D) structure, the image obtained by a gamma ray camera is a 2D view of the 3D distribution of a radionuclide. An important objective in nuclear medicine is to make SPECT more useful by utilizing and improving its quantitative capabilities. The availability of SPECT is well established. The cost-effectiveness of SPECT as a diagnostic imaging modality is demonstrated by the large number of installed systems worldwide. SPECT is an important clinical imaging modality and research tool, not only in the United States, but throughout the world. Consequently, the qualitative and quantitative improvement of SPECT has great clinical and research significance.
SPECT imaging is performed by using a gamma camera to acquire multiple 2-D images (projections), from multiple angles. SPECT scans typically are performed on general-purpose instruments with a collimator made of dense material such as lead. The collimator restricts the photons from the radioactive decay to come from certain lines of sight. A single view records photons for a period of time while the camera is motionless. The camera is then rotated by a few degrees and a second projection is recorded. From a large set (˜120) of projections, the 3D distribution of the radiolabeled pharmaceutical can be reconstructed through computer algorithms.
SPECT has many pharmaceuticals that are useful for imaging function of various organs. For example, cardiac imaging is performed to assess left ventricular function with gated radionuclide ventriculography, and to evaluate myocardial perfusion with agents such as thallium-201 and Tc-99m labeled compounds (Sestamibi, Tetrofosmin); I-123 labeled MIBG has been used as a method for measuring cardiac sympathetic innervation. Also, Biscisate, Sestamibi, HMPAO, IMP, and ECD can be used with SPECT for visualizing cerebral blood perfusion. TRODAT and other compounds are useful for imaging dopamine receptors for differential diagnosis of Parkinson's disease. Amyloid plaque imaging is used in the diagnosis of Alzheimer's disease. Tc-99m-labeled Sestamibi is also used to image breast cancer. Limb imaging is used for sarcoma, osteomyelitis (infection), and stress fractures.
A collimator is a device that filters a stream of photons so that only those traveling parallel to a specified direction are allowed through. Collimators are used in SPECT imaging because it is currently not possible to focus radiation with such short wavelengths into an image through the use of lenses as is routine with electromagnetic radiation at optical or near-optical wavelengths.
Without a collimator, rays from all directions would be recorded by the gamma ray camera; for example, gamma rays from the top of a specimen to be imaged may travel in both upward and downward directions. Thus, the image of the top of the specimen may be recorded at both the top and bottom of the gamma ray detector. Such an effect would occur for all parts of the specimen, resulting in an image so blurred and indistinct as to be useless.
When a collimator made of lead or other materials that absorb instead of pass gamma ray radiation is used, only gamma rays that are traveling nearly parallel to the openings in the collimator pass through the collimator to the gamma ray camera. Any other gamma rays are absorbed by hitting the collimator surface or the sides of an opening. This ensures that only gamma rays perpendicular to the gamma ray camera are recorded. In other words, gamma rays from the top of a specimen can only pass through the top of the collimator, thus ensuring that a clear image is produced.
Although collimators improve the resolution of the recorded image by blocking incoming radiation that would result in a blurred image, by necessity they also reduce the intensity (sensitivity) of the recorded image. In fact, most lead collimators let less than 1% of incident gamma rays through to the gamma ray camera.
The choice of collimator is a key decision in the quality of the resulting reconstruction. There are multiple types of existing collimators: parallel-beam, fan-beam, cone-beam, pinhole, and some other custom collimators. These collimators determine the trade-off between sensitivity (the number of recorded photons), the resolution (how well the line of a particular photon from the specimen to the gamma ray camera is known) and the field of view (the maximum size of the object to be imaged). Within a collimator family (e.g., parallel-beam), trade-offs are also possible, such as using longer lead holes to get better resolution at the cost of reduced sensitivity.
Cone-beam collimation has its best resolution near the collimator and its best sensitivity near the focal spot (i.e., far from the detector). Analogously, fan-beam has its best resolution near the collimator and its best sensitivity near the focal line. Fan beam collimation also offers the advantage of 2D complete-sampling using a circular orbit. However, the magnification is smaller, resulting in worse resolution. Although, single-pinhole collimation typically cannot offer 2D complete-sampling using a circular orbit, it has both its best resolution and sensitivity near the focal spot (aperture). Thus, single-pinhole is most advantageous when a small Radius of Rotation (ROR) may be achieved.
Thus, it is highly desirable to blend these techniques to create a collimator with more favorable characteristics for some imaging scenarios. In particular, it is highly desirable to develop a new collimator and collimation technique that combines the resolution and sensitivity properties of pinhole Single Photon Emission Computed Tomography (SPECT) imaging with the 2D complete-sampling properties of fan-beam collimation. Briefly, the possible advantages over single-pinhole SPECT for clinical use are: (i) increased sensitivity; (ii) improved complete-sampling properties; (iii) easier scan setup; and (iv) faster reconstruction times. The primary advantage over fan-beam is improved resolution and/or sensitivity.
Such an inventive collimator and collimation technique is herein referred to as a slit-slat collimator and a slit-slat collimation technique. When multiple slits are employed, such an inventive collimator and collimation technique is herein referred to as a multislit-slat collimator and a multislit-slat collimation technique